Initial setup for docs of python API (#6445)

Initial setup for docs of python API (#6445)

This PR adds the basic setup for including the python API in our sphinx documentation and shows how it works using the Datatype class as an example. In detail
- it enables sphinx.ext.autodoc and makes sure the generated pycvc5 is in the search path
- adds a index page for the python API
- adds a page for the Datatype class
- adds docstrings for the Datatype class
- does some finetuning (remove source locations, but adds signature information)